Egyptian authorities say 37 Morsi supporters died during an escape attempt on Sunday; divergent explanations have emerged for the deaths of the prisoners.

The U.S. State Department voiced deep concern on Monday about the deaths of Muslim Brotherhood prisoners while in custody in Egypt, terming them "suspicious," and made clear that it does not believe the Islamist group should be banned.

Almost 900 people, including nearly 100 soldiers and police, have died in Egypt since the authorities on Wednesday began to forcibly break up Muslim Brotherhood sit-ins by supporters of Morsi, who was toppled in a July 3 military coup.

Police have rounded up hundreds of Morsi's Brotherhood backers in recent days as the army-backed government has tried to end weeks of protests and to stamp their authority on the deeply polarized nation.

Divergent explanations have emerged for the deaths of the prisoners on Sunday.

A coroner's report said the men died from suffocation after police used teargas to stop a mass escape on Sunday while a group of more than 600 suspects were being transported to the Abu Zabal prison on the outskirts of Cairo.

Photos provided by the lawyers representing the detainees show dead bodies with charred faces and limbs and others covered in bruises which the lawyers said were signs of torture. Details of the incident remain unclear, they said.